This invention relates to new and useful improvements in pneumatic servo valves used with automatic web guiding systems.
Automatic web guiding systems are used to control the lateral position of moving webs of material, such as paper, plastics, textiles and other flexible materials being processed in continuous or roll form. A signal is generated by the position of the web edge relative to a sensor, and the signal output either directly controls an actuator connected to positioning means or indirectly controls an actuator through a servo valve which is connected to a web positioner.
Various types of servo valves have been incorporated into automatic web guiding systems to control the operation of actuator means, such as a spool type servo valve. The primary problem of the spool type pneumatic valve is the sticking due to friction and contamination from the air. In comparison to hydraulic valves, the pneumatic valve lacks the flushing and lubrication provided by the hydraulic fluid of a hydraulic valve.
Most all-pneumatic automatic web guiding systems have not used servo valves, but have operated their actuators directly with "feeler" type sensors that ride on the web edge, or an open vacuum type sensor which has one or more "intake" orifices supporting the web edge. Both systems are used in conjunction with spring-opposed single acting actuators.
A low pressure, non-contact pneumatic web sensor is highly desirable for sensing thin, delicate webs. Such a sensor requires a servo valve to provide the air pressure and flow necessary to operate an actuator for web controlling means.
The idea of a suspension valve is not new or unique. For example, Fluid Power Control, (Blackburn, Reethof, and Shearer, M.I.T. Press, 1960, p. 241.) describes a small suspension valve in which the flexure hinges, the stationary support block, and the moving block are all machined from a single block. A similar suspension valve is shown in a publication of American Society of Mechanical Engineers Transactions, Vol. 76 (1954) pp. 907, authored by Shih-Ying Lee. These devices utilize rigid suspension columns with cut-out portions which form hinges. The type of structure shown in Blackburn and Lee has severe flexure limitations and is limited and essentially unsuited as an automatic guiding system servo valve. Furthermore, the single block construction and portage configuration make this valve costly to manufacture and assemble. Other types of valves have been used such as shown in U.S. Pat. No. 3,674,236 which utilize flexible blades in tension that maintain a valve block in a position such that valve parts can be maintained in slidable, frictional, working engagement. This friction engagement for obvious reasons defeats use of this type of valve with a low pressure air signal system.